The present invention relates to a dielectric composition based on a mineral oil for electrical equipment having an improved gas absorption.
Mineral oils are widely used as insulating oils in various electrical devices, such as transformers, capacitors and cables.
These mineral oils comprise compounds of different natures, such as paraffin chains, saturated cyclic compounds, denoted as naphthenic compounds, or aromatic structures, including condensed polyaromatics.
Aromatic compounds bestow better dielectric properties, such as better breakdown voltage or so-called gassing properties, on the mineral oils.
The gas absorption is characteristic of the behaviour of an oil under a high electric field.
In the presence of gaseous hydrogen, the oil can either produce more gas, in which case it is said to be gas evolving, or can absorb the hydrogen; it is then said to be gas absorbing.
So-called gas absorbing oils are desired for electrical equipment.
Condensed polyaromatics are diverse and varied in nature but the majority are regarded as carcinogenic. Consequently, in order to reduce the content of polyaromatics in the mineral oils, the latter are subjected to a hydrogenation treatment. However, this treatment exhibits the disadvantage of resulting in the complete disappearance of the other aromatics regarded as not very toxic.
It has been discovered that the use of low amounts of compounds of the polyarylalkane family makes it possible to improve the gas absorption property of the mineral oils.
A subject-matter of the invention is therefore a dielectric composition for electrical devices, characterized in that it comprises from 99% to 70% by weight and preferably from 99% to 80% of a mineral oil and from 1% to 30% by weight and preferably from 1% to 20% of at least one polyarylalkane composition chosen from:
the compositions (I) comprising a mixture of products of formula (A): 
in which formula n1, and n2=0 or 1, which comprises products (A) such that n1+n2=0 and products (A) such that n1+n21, and of products of formula (B): 
the compositions (II) comprising a mixture of two products (C) and (D), in which mixture:
the product (C) is a mixture of isomers of formula: 
with p1 and p2=0, 1 and 2, and p1+p2xe2x89xa63, and
the product (D) is a mixture of isomers of formula: 
with pxe2x80x21, pxe2x80x31 and p4=0, 1 and 2 pxe2x80x22, pxe2x80x32, p3 and p5=0 and 1, pxe2x80x21+pxe2x80x31+pxe2x80x22+pxe2x80x32+p3+pxe2x80x23+p4+p5xe2x89xa62,
the compositions (III) comprising a mixture of two products (A1) and (A2), such that:
the product (A1) is a mixture of isomers of formula: 
with m1 and m2=0, 1 or 2, and m1+m2xe2x89xa63,
the product (A2) is a mixture of isomers of formula: 
with q1 and q2=0, 1 or 2, and q1+q2xe2x89xa63,
at least one of the compounds (A1) and (A2) comprising an isomer having three benzene nuclei,
the compositions (IV) comprising the two products (A1) and (A2) and, in addition, at least one compound chosen from the following products (E1), (E2) or (E3):
(E1) is an isomer or a mixture of isomers of formula: 
with rxe2x80x21, rxe2x80x31 and r4=0, 1 or 2 rxe2x80x22, rxe2x80x32, r3, rxe2x80x23 and r5=0 and 1, and, rxe2x80x21+rxe2x80x31+rxe2x80x22+rxe2x80x32+r3+rxe2x80x23+r4+r5 is less than or equal to 2, R1 and R2 represent a hydrogen atom,
(E2) is an isomer or a mixture of isomers with the same general formula as (E1), except that R1 and R2 represent a methyl and the coefficients r are replaced by s and have the same meaning,
(E3) is an isomer or a mixture of isomers with the same general formula as (E1), except that R1 and R2 are different and represent a hydrogen atom or a methyl radical and the coefficients r are replaced by t and have the same meaning.
According to the present invention, the compositions (I) can comprise product (A) comprising 2 nuclei, (methylbenzyl)xylene, and product (A) comprising 3 nuclei, which is denoted by bis(methylbenzyl)xylene. This product (A) comprising 3 nuclei can be product such that n1=1 and n2=0, product such that n1=0 and n2=1, or a mixture of these last two. The polyarylalkane composition can also comprise products such that n1=1 and n2=1.
Mention will be made, by way of illustration of compositions (I) which can be used according to the present invention, of the polyarylalkane composition sold by the Company Elf Atochem S.A. under the name Jarisol XX, which has a content by weight of compounds comprising 2 and 3 aromatic nuclei of greater than 99%.
Mention will be made, by way of illustration of compounds (II) which can be used according to the present invention, of the polyarylalkane composition sold by the Company Elf Atochem S.A. under the name Jarylec C100, which is composed essentially of 70% to 80% by weight of a mixture of benzyltoluene isomers (product (C), p1=p2=0) and of 20% to 30% by weight of dibenzyltoluene isomers (product (C), p1=1, p2=0 or p1=0 and p2=1) and of ditolylphenylmethane isomers (product (D), pxe2x80x21+pxe2x80x31+pxe2x80x22+pxe2x80x32+p3+pxe2x80x23+p4+p5=0).
These compositions can be obtained by processes disclosed in Patents EP 136,230-B1, EP 299,867-B1, EP 384,818-B1 and EP 500,435-B1, incorporated in the present invention by reference, which consist in chlorinating toluene or xylene and then carrying out a condensation of Friedel-Crafts type either on toluene or on xylene (mixture of isomers) or on a toluene and xylene mixture or on benzene or on a benzene and toluene mixture. On completion of the reaction, the unconverted reactant or reactants is/are removed directly by distillation and then the crude product can be subjected to a dechlorination treatment, such as disclosed in Patent EP 306,398-B1.
Thus, for example, the compositions (II) can be obtained by a process disclosed in Patent EP 136,230-B1 which consists, in a first stage, in reacting chlorine with toluene by a radical reaction in the presence of a free radical generator at a temperature of between 50xc2x0 C. and 110xc2x0 C. and then, in a second stage, the reaction product from the first stage is subjected to a condensation reaction with toluene in the presence of FeCl3 at a temperature of between 50xc2x0 C. and 100xc2x0 C.
The compositions (I) can be obtained according to a process disclosed in Patent EP 050,435-B1 which consists in condensing (methyl)benzyl chloride with xylene in the presence of FeCl3.
The dielectric compositions according to the invention exhibit the advantage of having an improved gaseous behaviour (improved gassing).